Technical Field
The present invention relates to an image inspection system and an image inspection method, and more particularly to setting of inspection thresholds used for determining defects in image.
Background Art
Conventional inspections of printed matter such as printed papers are conducted by visual inspection, but inspection apparatuses have been introduced to conduct the inspections as a post-processing operation of the offset printing. As for the inspection apparatus, the printed matters are visually inspected by an operator to select a printed matter having satisfactory image quality, and then the selected printed matter is scanned to generate a master image to be used as a reference image. The master image and inspection target printed matter are compared with each other by scanning the inspection target printed matter, and based on difference between the master image and the inspection target, defect of the inspection target printed matter can be determined.
However, the printing apparatuses of digital to press such as image forming apparatuses using electrophotography typically print images with a small volume, and also print different images for each page (i.e. variable printing), in which generating a master image from printed matter as a reference image is not efficient. In this type of image forming apparatuses, the master image can be generated from print data to efficiently conduct the inspection for the variable printing.
In this image inspection process, the defect of printed matter can be determined based on the above mentioned difference level. Specifically, scanned images prepared by scanning sheets printed with images and the master image generated from the print data are compared, in which positions and sizes of comparing images are matched and then the comparing images are compared for each pixel based on a given threshold.
JP-2008-003876-A discloses an image inspection process for an inkjet printer, which can verify inspection precision for the image inspection. Specifically, defects that may likely occur for the inkjet printer are artificially printed on sheets, the sheets having printed with the artificial defects are inspected, and then it is verified whether the inspection can be conducted effectively.
The above mentioned threshold used for comparing the images affect the inspection precision, therefore effective thresholds need to be set for high precision inspection. JP-2008-003876-A discloses a configuration to determine whether the inspection is conducted effectively using thresholds such as thresholds set in advance, in which suitable thresholds are not set automatically.